In recent years, for the purpose of achieving prevention of leakage of information from a computer terminal device and facilitation of management of applications on the terminal, a thin client system has been introduced. A thin client system makes a server device execute all applications inputted into, outputted from and displayed at a terminal device and also makes the server device manage all generated files relating to the execution.
In such a thin client system, drawing data of an application program executed by the server device is transferred to a client terminal device through a network such as a LAN, and displayed on the screen of the terminal device. Therefore, when the amount of drawing data to be processed increases or when the number of terminal devices connected to the server device increases, the load on the server device for transferring the drawing data to the terminal devices increases, and problems such as deterioration of response time and considerable limitation of the number of connected terminal devices arise.
Thus, a method of encoding and transmitting image data of a rectangular region including an updated region in an image is proposed as a first related technique relating to the present invention (for example, see Patent Document 1). According to this first related technique, in a case where an updated region in an image is only a partial region, the need for encoding and transmitting the whole image is eliminated, and encoding and transmitting only image data of the partial region is needed. Consequently, the amount of data to be transmitted can be reduced.
On the other hand, it is known to, when transmitting and/or receiving image data via a communication network, encode the image data in order to compress the amount of the data (for example, see Patent Document 2).
When irreversible transformation such as quantization is used in data compression, the quality of an image decoded on the reception side may deteriorate as compared with the quality of the original image on the transmission side. Also, in a case where a data loss or a transmission error occurs in a communication path and so on, the quality of an image on the reception side may deteriorate. In order to suppress propagation of image deterioration to a succeeding frame, resulting from the abovementioned things, a method of periodically inserting a signal of intra update (intra refresh) when transmitting a video image is devised. In general, in intra update, a method of dividing a frame into a plurality of partial regions (tiles) and transmitting little by little is used because encoding the whole frame leads to considerable increase of the amount of codes.
For example, it is proposed as a third related technique relating to the present invention to, after transmitting the codes of an updated region, perform intra update while circulating among low-quality tiles, within the range of a surplus amount of codes that can be transmitted (for example, see Patent Document 3). Consequently, even when the area of a drawing updated region of a frame varies largely, it is possible, by changing the area of a target region for intra update, to achieve a high image quality while sufficiently utilizing the surplus band. For example, with respect to a frame with no difference or a frame in which the area of a drawing updated region is small, it is possible to perform intra update through the most tiles that can be held in the surplus band. Moreover, by devising to preferentially circulate among tiles of a region encoded with low image quality or tiles having not been refreshed for a long period, it is possible to shorten a period that the low-quality tiles remain in the screen.
On the other hand, it is proposed as a fourth related technique relating to the present invention to encode an updated region after undergoing motion compensation as an image (for example, see Patent Document 4).
Further, it is proposed as a fifth related technique relating to the present invention to encode a block including a character region with high image quality (for example, see Patent Document 5). The fifth related technique reduces deterioration of the quality of character information by updating a quantization coefficient in a character region which requires sharpness in a moving image, separately from a quantization coefficient in the other region, so that the quantization coefficient in the character region becomes equal to or less than a predetermined threshold when the quantization coefficient in the character region is larger than the threshold.
Further, it is proposed as a sixth related technique relating to the present invention to read out a video signal per macroblock from a frame buffer and, when the while level is high in this macroblock, when data exists in an adjacent block, or when the result from a median filter shows continuity, determine as a caption (a character region) (for example, see Patent Document 6).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-085502    Patent Document 2: Japanese Patent Publication No. 004035475    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2010-206753    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2009-194627    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2002-204459    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2005-197879
In a case where the upper limit of the amount of outputted codes per frame is fixed, with respect to a frame in which the area of an updated region is large, it is difficult to encode the updated region with high quality, and therefore, there is no choice but to encode the updated region with low quality. For the region encoded with low quality and transmitted, intra update is performed while circulation among the low-quality tiles is performed within the range of a surplus amount of codes that can be transmitted in the following frames. In the infra update, as described in the third related technique, a tile which should be retransmitted with high quality is selected based on the quality of a tile and a period that a tile is not refreshed. Therefore, when the retransmission candidate tile includes a photograph region which is lower-quality than a character region or a photograph region which has not been refreshed for a longer period than a character region, a case where the photograph region is preferentially selected and the character region is left low-quality for a long time occurs.
However, in general, the level of quality has a larger effect on character information than on photograph information. This is because the meaning of character information largely varies depending on a difference in shape of the detail, whereas the meaning of photograph information can be roughly understood based on the outline alone.
As mentioned above, it is proposed as the fifth related technique relating to the present invention to encode a block including a character region with high quality. Thus, it is possible to apply the fifth related technique to the third related technique relating to the present invention and, when encoding an updated region, make the encoding quality in a character region higher than in the other region. However, in a case where an updated region is large in size and includes character regions at the high ratio, it is impossible, when the amount of all codes of the updated region exceeds the upper limit of an allowable code amount per frame, to achieve a high image quality in all of the character regions. As a result, in the same manner as mentioned above, a case where a character region is left low-quality for a long time occurs, and the legibility of characters in a video image decreases.